This invention relates to power-supply apparatus in which generators such as fuel cells, solar cells, particularly solar photovoltaic cells, magneto hydrodynamic generators or the like supply power to a load. The invention has particular relationship to such apparatus which includes a storage battery for storing energy from the generator when the output of the generator exceeds the loading or for supplementing the energy supplied by the generator when the loading exceeds the output of the generator.
A dramatic increase in the use of the above-mentioned generators is anticipated during the next few years. In particular it is expected that there will be wide use of small power supply apparatus of this type for private residences. Such small power-supply apparatus may include generators of the different types mentioned above but predominantly the generators will be of the solar photovoltaic type. Basically this apparatus includes the generator, typically a photovoltaic array, and a power conditioner; i.e. an inverter (DC-to-AC converter). It is contemplated that normally the residential power-supply apparatus will supplement the power supplied by a public utility. But in some instance operation independently of the utility will be necessary, or, as in some areas in Saudi Arabia, where such generator apparatus is in use, power from a public utility will not be available at all. In such power-supply installations a storage battery in addition to the generator and the inverter is necessary. There is need for an interface between the generator and the battery in such installations. This necessity arises predominantly from the voltage differential which exists at the output of the generator. In the case of a solar photovoltaic generator array such differential may be caused by differences in the light incident on the array on different days and at different times of any day. In the absence of an interface full use of the energy from the generator is difficult, if not impossible. The interface is also necessary because of differences in the loading of the generator.
Attempts, in accordance with the teachings of the prior art, have been made to overcome the problems raised by the variation of the power or voltage output of the generator and the load variations. It has been proposed that the battery be connected directly across the output of the generator (through blocking diodes). This approach dispenses with the interface altogether and has the disadvantages mentioned above. A second proposal is to interpose, between the generator and the battery, a DC-to-DC converter; i.e., a converter having the full rating of the generator. This proposal is costly. It is also inefficient because the interface converter absorbs a substantial portion of the generator power. A further proposal is to connect a full-rated auxiliary inverter (DC-to-AC converter) between the battery and the input to the load; i.e., between the battery output and the AC output of the main inverter connected to the generator. This proposal is an improvement over the second proposal as regards the energy delivered from the generator to the load but it suffers from the same disadvantages as the first proposal. Its cost is high and its efficiency is low.
It is an object of this invention to overcome the disadvantages and drawbacks of the prior art and to provide power supply apparatus of the type described above that shall effectively and efficiently and at low cost permit voltage differentials to exist between the storage battery and the generator while permitting controlled current to flow into or out of the battery.